In some prior art BTM systems these riser connections and ESD valves systems are placed in an enclosed space within the turret near the structural connector of the BTM. To avoid the risk of an explosion in these enclosed spaces, ventilation of the enclosed spaces is needed for example via ventilation shafts and forced air. The solution of using an inert gas in the enclosed space can also avoid the risk of an explosion but the space can not be entered for maintenance and repair activities or in emergency situations. Furthermore to have an enclosed turret space under water also creates the danger of ingress of water due to leakage of (damaged) seals between buoy and turret.
To avoid the above-mentioned problems it is known in the prior art to pull up each riser separately through the buoy up through the rotatable turret to the vessel deck, so that the riser connections are in an open and natural ventilated environment as is shown in U.S. Pat. No. 5,316,509 in the name of Sofec. This publication shows a BTM system with vertical displaceable risers in a turret which is rotatably supported in a moon pool of a vessel via a bearing system.
The patent publication WO9324733 in the name of Statoil shows a submerged buoy which is provided with a weight-carrying main bearing so that it can rotate around its own axis when it is connected to the bottom of a vessel. This publication also shows a riser deck with riser connectors and valves which can be pulled up to deck level and which can be lowered onto the buoy before the buoy is disconnected from the vessel. This design can be a solution for a limited amount of risers as the axial radial bearing of the buoy can only be fabricated to a certain maximum diameter and because the vertical displacement of one large diameter deck within the rolling vessel creates large problems related to the guiding system. It is anyway from a maintenance point of view, not preferred to have a buoy concept with an expensive weight-carrying bearing which lies in the seawater.
Patent publication WO8808806 also shows a movable deck to which risers and valves are connected. The deck is pulled up only to tension the attached vertically tensioned risers from the BTM systems as these systems which differ are provided with flexible curved risers. The deck can be lowered onto a very large size buoy of which the top is above water level when it is connected to a vessel via a turret which has a reduced height. This design does not provide a quick disconnectable system as the large buoy has to be flooded with water before it is able to sink out of the turret-moon pool. There is also not one central single structural connector that can be opened to quickly disconnected the buoy from the turret.